Bernard Tomic admitted to suffering stage fright but denied not trying after suffering a humbling three-set belting at the hands of home hero Andy Roddick at the US Open in New York.

Tennis great John McEnroe accused Tomic of not having a genuine go in the final set of his 6-3,6-4, 6-0 second-round defeat on Saturday (AEST).

"Tomic is teeing it up. It looks like the tank job," McEnroe said while commentating on prime-time TV in America.

"This is a shame. You don't like to see this. I like to see Andy win but, other than that, it's poor."

When Roddick closed out the 22-minute final set - in which Tomic won only five points - with an ace, McEnroe added: "A well-deserved beating."

Tomic, though, insisted he was simply overwhelmed by the occasion.

The 19-year-old was playing in the sold-out Arthur Ashe Stadium - the world's largest tennis arena which seats 22,500 spectators - for the first time and on the night after Roddick announced his retirement.

"I was a bit nervous the first set. It's very difficult the first time to be in front of 22,000 people," Tomic said.

"It's very different. A different feeling playing on the back courts than playing on the biggest stage in the world, biggest country in the world.

"I wasn't looking up, that's for sure. More I looked up, the more I realised how much people were there."

Tomic felt Roddick was untouchable.

"I couldn't get the racquet on the ball," he said.

"Andy was playing, I think, really good at the net, coming into the net.

"Every time I wanted to pass him, he ended up hitting a half-volley winner or a volley winner.

"He was on top of me the whole match. I can't complain. The whole match was his way from start to finish pretty much."

The 19-year-old reacted angrily when asked about McEnroe's tanking allegation.

"Really? What do you think?" Tomic asked his inquisitor.

"That's how I play. Do you have a problem with that?"

He maintained it was merely a suffocating experience.

"To play one of the biggest players in America, I was thinking about it too much leading into the match," Tomic said.

"He was serving well. So I just couldn't find my way to get out of that little zone.

"I feel like I needed to hit 10 minutes on the court. It's difficult. The top seeds always get the opportunity to hit on that court.

"Yeah, this is his home city where he plays the best. I wasn't quite comfortable I think the whole match on that court. It was very strange."

Roddick, the 2003 champion and one-time world number one, next meets Italian Fabio Fognini for a spot in the last 16.

"I'm going to look forward to it and I'm going to try and stick around a bit longer," Roddick said.